Ceramic technology is in large part an empirical art. The reason is that the complex systems used are not sufficiently understood to allow the effects of changes to be predicted or controlled. In the electronics industry, an aluminum oxide ceramic is commonly used as a substrate for electrical circuits. Typically, the circuit metallization pattern is effected by stencil or screen printing the desired pattern using a paste made of metal oxides, organic and inorganic binders, and other carriers. The paste is then dried and fired so that the resulting pattern consists of conductive metal particles. This method suffers from being slow and costly, and is severely limited in the degree of resolution achievable.
The second most common method is deposition of a thin film of metal on the ceramic using vacuum techniques such as evaporation or sputtering. The desired metal pattern is fabricated using photolithography and plating techniques and the undesired thin film metal is etched away. While this method is capable of significantly improved resolution as compared to the screen printing method, very few metals will adhere properly to the ceramic, and require special conditions in order to be successful.
Conventional thin film technology requires that a first metal, known as the `glue`, be deposited onto the ceramic surface. The mechanism of bonding of the glue metal to the ceramic is not thoroughly understood, but is thought to be via a bond between oxygen atoms in the ceramic and the deposited metal atom, thereby creating a strong metal-oxide bond between the metal and the ceramic. Metals used as the `glue` typically have the property of extremely rapid oxide formation (for example, chrome, nickel, aluminum, titanium). Other metals, such as copper, cannot be successfully utilized as a `glue` metal because the copper-oxygen reaction rate is very slow, and copper does not form the required metal-oxide bond. In order for proper ceramic-metal bonding to occur, it is imperative that the metal be deposited in pure form in an oxygen-free environment. Trace amounts of oxygen will rapidly convert the reactive metal to an oxide during deposition and prevent proper bonding to the substrate.
Once successful bonding of the `glue` metal to the ceramic has occurred, the `glue` metal must be maintained in an oxide free form in order to insure proper bonding of subsequent metal layers to the `glue` metal. As in the previous interface, oxides will prevent proper adhesion between metal layers. In electronic applications, copper is typically deposited over the `glue` metal in order to provide a relatively non-reactive surface that can be subjected to further processing, since the rate of oxide formation of copper is significantly less than that of the `glue` metal.
The conventional method of depositing thin films of metal onto ceramic suffers from:
(1) the need to use a `glue` metal,
(2) the need to have all traces of oxygen excluded from the deposition environment, and
(3) the relative paucity of adequate `glue` metals.
Clearly a process that eliminates the `glue` metal and requires less stringent control over the oxygen environment would be desirable.